Apparantly the forum/server has spontaneously switched to using https, and because of that the script's include parameter no longer matches. You can edit the script and add // @include https://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=*;* (i.e. same line but with https iso http) And then it works again (for me, in any case).

(Some other things remain broken though, because of mismatched protocols. Like the YABBC tag buttons and inserting smileys by clicking them)

I've updated the Math Symbols script with the https fix towr mentions, and also added functionality to restore the YABBC tags and Smileys buttons. I've only tested it under modern Firefox, but I expect it to work anywhere user scripts are supported.

I really should update the Howto at the top of this thread one of these years...

Uses http rather than https for the img tags, since at least for me using Tampermonkey on 64-bit Chrome it was broken. Frankly I'm a bit surprised it still works at all more than a decade after I first wrote it!